In recent years, a mold press molding method is widely used as a production method of an optical lens, having high productivity and low production cost. In this production method, a droplet-shaped preform glass heated to a temperature higher than a glass transition temperature is pressed using a pair of upper and lower molds each having a high precision surface, thereby realizing an optical lens having a desired shape.
In general, in a mold press production method, expensive release films are formed on the surfaces of molds in many cases in order to prevent fusion between the mold and a heated glass. However, those release films are liable to deteriorate when exposed to high temperature. As a result, the durability of the mold is decreased, and manufacturing costs are increased. To improve durability of a mold, an optical glass having a low yield point (Ts) and low-temperature softening properties is necessary.
The mold press molding method mostly uses a droplet-shaped preform glass obtained by cooling a dropped glass melt. However, in order to increase production yield of a preform glass, it is necessary to prevent devitrification of a glass during dropping through molding, which is the cause of yield lowering, and optical glass is also required to have a low liquidus temperature (L.T.).
In recent years, a so-called middle refractive index and low dispersion optical lens having a refractive index (nd) of from 1.55 to 1.65 and an Abbe number (νd) of from 55 to 65 is generally used in optical pickup of various optical disc systems such as CD and DVD, video cameras, digital cameras and the like. Demands of those commercial products are remarkably increasing. With the demands, an inexpensive and high quality optical glass for middle refractive index and low dispersion suitable for a press molding method is desired.
Barium crown glass type or heavy crown glass type glasses have conventionally been known as a middle refractive index and low dispersion glass having a refractive index (nd) of from 1.55 to 1.65 and an Abbe number (νd) of from 55 to 65. For example, many glasses of SK (SCHOTT AG, catalog name) type are described in “Glass Composition Data Book, 1991, The Glass Manufacturer's Association of Japan”. However, those glasses generally have a high yield point exceeding 600° C., and are therefore not suitable for mold press molding.
To overcome this problem, it has been investigated to add an alkali oxide such as Li2O, Na2O or K2O to those glasses. However, there is the problem that SiO2—B2O3—BaO—R2O glass obtained by merely adding only an alkali oxide to the conventional SiO2—B2O3—BaO glass has poor weather resistance.
In view of the above, addition of weather resistance improvement components as described below has been investigated. However, to realize practically sufficient weather resistance, the component must be added in a large amount, and as a result, there is the problem that other properties such as devitrification resistance and optical properties deteriorate. For example, Patent Document 1 proposes a glass comprising SiO2—B2O3—BaO-R2O glass and added thereto, TiO2 and Nb2O5. However, due to the addition of TiO2 and Nb2O5 in large amounts, low dispersibility of refractive index cannot be realized. Furthermore, Patent Document 2 proposes an optical glass comprising SiO2—B2O3—BaO—R2O glass and added thereto in a large amount, Gd2O3. However, there are the problems that devitrification resistance properties markedly deteriorate, and due to the addition of a large amount of Gd2O3 which is an expensive material, it increases costs.
On the other hand, Patent Document 3 proposes SiO2—B2O3—SrO glass using SrO in place of BaO as an alkali earth oxide. This glass has a low yield point (Ts) and excellent weather resistance, and regarding optical properties, it has middle refractive index and low dispersion properties. However, there are the problems that a liquidus temperature (L.T.) is relatively high, and it is difficult to prepare a preform by a falling-drop method. A middle refractive index and low dispersion optical glass which has overcome those problems has been desired.
Patent Document 1: JP-A-6-107425
Patent Document 2: JP-A-2004-2178
Patent Document 3: JP-A-7-149536